Ion rocket engine



Dec. 19, 1961 K. w. EHLERS ETAL 3,014,154

ION ROCKET ENGINE Filed Oct. 1, 1959 2 Sheets-Sheet-l THRUST POWER SUPPLY INVENTORS. KENNETH W. EHLERS FERDINAND VOELKER ATTORNEY.

9, 1961 K. w. EHLER-S ETAL ,01

ION ROCKET ENGINE Filed Oct. 1, 1959 2 Sheets-Sheet2 POWER SUPPLY GAS FLOW POWER SUPPLY -2,ooov

GAS FLOW 26 54 ,p INVENTORS. 000V KENNETH W. EHLERS 6 Z'OOOV BY FERDINAND VOELKER s SUPPLY fiw wm ATTORNEY.

charged ions.

United States Patent Ofilice Patented Dec. 19, 1961 3,014,154 ION ROCKET ENGINE Kenneth W. Ehlers, Lafayette, and Ferdinand Voelker III,

Concord, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Oct. 1, 1959, Ser. No. 843,898 15 Claims. (Cl. 315-111) as is required for escaping from the gravitational field of the earth. In contrast, the system of the present invention efficiently provides a comparatively low thrust over a very long time period as is required for extended trips which might last for an interval of months or years. Thus the present invention will be most eifectively utilized on space vehicles launched to distant points from orbiting satellites or other space platforms outside the immediate vicinity of the earths gravitational field.

An ion rocket is somewhat comparable to an ion source having special additional provision for accelerating and ejecting ions to generate thrust.

To obtain the necessary magnitude of thrust for a rocket frame of practical mass, a very high ion current is required as compared to that produced by most conventional ion sources. In the present invention there is provided a unique ion generating structure characterized by a large surface area nearly all of which emits ions. Accelerating electrodes are disposed adjacent the ion emitting surface for imparting a high velocity to the ions to produce thrust. The 'invention is provided with novel means for overcoming a further problem characteristic of ion rockets, specifical- "ly that of the accumulated negative electrical charge on the rocket frame created by the emission of positively In the absence of corrective provision such negative charge attracts any nearby charged particles or bodies, notably including the ions ejected from the rocket, which attraction creates a drag on the motion of the rocket frame and counteracts the thrust. The present invention includes a novel electron ejecting means for neutralizing the charge on the ions after the ions have been accelerated. An advantageous property of such means is that the number of electrons ejected for neutralization automatically equates with the positive charge of the ejected ions so that the net resultant charge on the rocket frame remains neutral and unchanged.

Although various substances are usable as a source of ions, cesium is a very satisfactory propellant from the viewpoint of having the lowest ionization potential of all the elements, i.e., 3.9 volts. In the invention the propellant is heated to a gaseous state and subsequently contacted with a heated metal of the group having a work .and very high efficiency results.

It is therefore an object of the present invention to provide a rocket engine utilizing ions as a propellant.

It is an object of this invention to provide a thrust generating system particularly adapted for space vehicles which are to be used for prolonged travel outside the terrestrial gravitational field.

It is a further object of the invention to provide a rocket engine using an ionized propellant in which substantially 100% of the propellant substance is expended usefully.

It is another object of this invention to provide an automatic means for maintaining a constant electrical potential on a rocket of the class using electrically charged particles as a propellant.

It is an object of the invention to provide an improved ionizing mechanism for use in ionic rocket propulsion systems.

Still a further object of this invention is to provide a self-regulated mechanism for neutralizing an ion beam emitted from an ion rocket.

It is another object of this invention to provide an ion beam generating system for an ion rocket which system emits ions over a very broad surface area.

The invention both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in conjunction with the accompanying drawing, in which:

FIGURE 1 is a perspective view of a rocket engine and nacelle structure therefor with portions of the nacelle being broken out to expose the engine components;

FIGURE 2 is an enlarged view of the portion of FIG- URE 1 enclosed by line 2 thereon, such view showing a typical section of the ionizing, accelerating, and neutralizr ing element of the engine;

FIGURE 3 is a sectional view taken along line 33 of FIGURE 2;

FIGURE 4 is a perspective view of a first modification of the structure shown in FIGURE 3; and

FIGURE 5 is a perspective view of a second modification of the structure shown in FIGURE 3.

Referring now to the drawing and, more particularly, to FIGURE 1 thereof, there is shown a nacelle or outer protective shell 10 with a generally ellipsoidal shape and a rectangular opening 11 at the rearward surface. The shell 10 need not be designed in accordance with conventional aerodynamical principles in those cases where the rocket is to be operated outside the terrestrial atmosphere, the ellipsoidal shape of this embodiment being simply a convenient form for enclosing the components and for providing cargo space with a minimum mass of material. A propellant is contained within a cylindrical vessel 12 situated within the shell 10 forwardly from opening 11, electrical heating coils 13 being disposed around the vessel 12 in coaxial relationship thereto and in thermal contact therewith to vaporize and pressurize the fuel. As will be hereinafter discussed in more detail, the element cesium is an example of a suitable propellant. A current source 14 for energizing the heating coils 13 is carried within shell 10 and may comprise a nuclear reactor operated generator, conventional batteries, solar batteries or any other suitable power supply.

Disposed rearwardly from vessel 12 and forwardly from opening 11 is a rectangular manifold 17 conforming in outline to the opening. An inlet pipe 16 connects the forward face of the manifold 17 with propellant vessel 12 and a plurality of outlet pipes 18 extend rearwardly from the manifold to a beam generating assembly 19 of rectangular configuration which assembly is secured within the opening 11 and which forms the rearward surface of the rocket. Assembly 19 which functions to ionize, accelerate, and electrically neutralize the propellant 3 is comprised of an array of contiguous like rectangular sections 20, each of which is supplied by an individual one of the manifold outlet pipes 18. Each manifold outlet pipe 18 contains a valve for control of the amount of propellant provided to each section 20.

Referring now to FIGURE 2, the detailed structure of a typical one of the sections 20 of assembly 19 is shown. Pipe 18 from manifold 17 communicates with a rectangular chamber 21 formed by a square gas-impervious forward wall 22 to which side and top walls 23 are secured. The rearward face ofthe chamber 21 is defined by a porous electrode 24 of conforming rectangular shape. As will hereinafter be discussed, the electrode 24 is preferably tungsten having a porosity of approximately 20% of the total volume, such porosity comprising minute channels communicating from chamber 21 to the rearward surface of the electrode.

Referring now to FIGURES 2 and 3 in conjunction, the outer surface of the porous electrode 24 is shaped into a plurality of transverse parallel channels or grooves 26 of concave cross-section. Pressure created by the heating of the propellant in the vessel 12 thus forces the gas from the chamber 21 through the porosities in the porous electrode 24 to the concave surfaces of channels 26.

If the porous electrode 24 is heated to approximately 1300 centigrade when used with cesium fuel, the heated electrode 24 ionizes nearly all of the atoms in the cesium vapor. This results from the porosity of the tungsten which provides numerous tortuous paths and insures that virtually all the cesium atoms contact the heated electrode 24. The mechanism of ionization by contact with heated tungsten is discussed in detail in an article entitled Ionization on 'Platinum and Tungsten Surfaces, by Datz and Taylor, Journal of Chemical Physics, September 1956, Volume 25, Number 3.

To effect heating of the electrode 24, a plurality of bores 27 extend transversely therethrough, one situated between each adjacent pair of the grooves 26. A rod shaped electrical heating element 28, is disposed in each such bore 27 and a power supply 29 is provided to supply energizing current therefor. Supply 29 may, if desired, be the supply 14 of FIGURE 1, but is a multi-tap D.C. source in this embodiment to supply the various potentials which will hereinafter be specified.

To accelerate ions emerging from the grooves 26 of electrode 24, an electrical field is established between such electrode and a plurality of transverse accelerating electrodes 31 disposed rearwardly from electrode 24 and aligned with the junctures between adjoining concave grooves 26. The accelerating electrodes 31 are semicylindrical in cross-section with the open side facing away from the porous electrode 24. A pair of dielectric slats 32 are secured to the sidewalls 23 of chamber 21 and form an extension thereof to support the ends of the electrodes 31 and for electrical isolation thereof. A negative potential, from an appropriate tap on power supply 29, in the order of 2000 volts relative to porous electrode 24 is applied to each accelerating electrode 31. The power supply 29 connection to the rocket frame is indicated by the ground symbol. There is thus established an electric field as indicated by dotted lines 33 which field is shaped by the described configuration of the accelerating electrodes 31 and the porous electrode 24 so that ions are bunched into a series of rearwardly directed beams of the general configuration indicated by dashed lines 34. Each beam 34 passes between a pair of accelerating electrodes 31 and into the space beyond. The accelerated ions thus provide a driving force in accordance with Newtons Third Law of Motion, producing the desired thrust on the rocket frame.

In order for a neutral charge to be maintained on the rocket it is necessary to remove from the rocket frame a number of electrons equal to the number of electrons removed from the propellant atoms in the ionization process in the porous electrode 24. Thus, if cesium is the propellant, it is necessary to remove one electron with each cesium ion emitted. Accordingly, a further element of the invention is a means for emitting electrons with automatic regulation of the distributed charge on the rocket frame to essentially a neutral value. The neutralzing apparatus comprises a rod shaped electron emitting cathode 35 heated by a filament 36, the combination disposed axially Within each accelerating electrode 31 and secured at the ends by the supports 32. The filament 36 is electrically heated by connection with an appropriate tap on the power supply 29, and with the cathode 35 is maintained at the electrical potential of the accelerating electrode 31 by a connection thereto. As a variation, the filament 36 may be used alone without a cathode 35. The cathode 35 emits a copious supply of electrons which are attracted toward the ion beam 34, as indicated by dashed lines 37 in FIGURE 3, and neutralize the positive charge thereof. The semi-cylindrical configuration of the accelerating electrode 31 shields the ions in the beam 34 from the electrons until full ion acceleration has occurred since premature neutralization of an ion would prevent the ion from attaining the maximum possible velocity. Automatic regulation of the rocket frame charge at near neutral value is now inherent in the de-ionization system since any buildup of negative charge on the rocket results in more electrons being attracted toward the positive ion beam 34 thereby returning the rocket frame charge to a less negative value.

Referring now to FIGURE 4, an advantageous modification of the structure of FIGURES 2 and 3 is shown in which additional means are provided to further prevent the impingement of ions on the accelerating electodes. In this embodiment the porous electrode 24 has the same configuration as that previously described and an accelerating electrode 41 with a minus 2000 volt potential is positioned rearwardly from the porous electrode 24 in essentially the same relationship as described for the first embodiment. The electrode 41, however, has a U-shaped cross-sectional configuration, the open side being rearmost. As before, a rod shaped filament 36 and cathode 35 are disposed within the accelerating electrode 41 and emit electrons which are shielded from the ions until the latter have been fully accelerated. Disposed against the outer surface of each arm of each accelerating electrode 41 is a permanent bar magnet 42 which magnets extend along the entire length of the electrodes and which have opposite poles along the front and rear edges. With the addition of the described form of bar magnets 42, a magnetic field, indicated by dashed lines 43, is created which will reduce ion loss by providing an additional focusing force on the ion beam. The ions tend to follow the magnetic lines and are thus guided away from the accelerating electrode 41.

In each of the embodiments of the invention heretofore described, it will be apparent that some electrons from the filament 36 are lost by being attracted directly to the accelerating electrode 31 or 41, or to the porous electrode 24, thereby causing an undesirable power loss.

Referring now to FIGURE 5, there is shown still a further modification of the ionizing and accelerating structure which eliminates the specified power loss and which in addition does not require filament power for the production of electrons, this embodiment thus constituting a highly efficient form of the invention. The porous electrode 24 is similar to that of the first two embodiments and is provided with heating elements 28 in a like manner. Transverse accelerating electrodes 51 are spaced rearwardly from electrode 24 and are aligned with the junctures between adjacent concave grooves 26 as before. The accelerating electrodes 51, however, are bars having a rectangular cross-section. A potential in the order of minus 50,000 volts is applied to the accelerating electrodes 51 from a power supply 54. -As in the previously described embodiments, ions emerging from electrode =24 are acceleratedto a high velocity by the high potential on the accelerating electrodes 51.

To provide for neutralization'of the ions without the use of a filament, av plurality of decelerating electrodes 52 are positioned parallel with the accelerating electrodes, the elements 52, preferably having a trapezoidal cross-sec tion which tapers to a smaller dimension at the rearmost edge. A potential of minus 2000 volts is applied to the decelerating electrodes 52 from the power supply 54, the resultant field acting to decelerate the ions and to return the energy thereof to the electrical system. Using the stated potentials, the final ion velocity is that which would have been attained solely with a minus 2000 volt accelerating potential as used in the previously described embodiments. The surfaces of the decelerating electrodes 52 which are exposed to the ion beam 53 are comprised of a material having high secondary electron emission qualities. After the ion beam 53 is focused and accelerated by the high negative potential on the accelerating electrode -1, a defocusing effect occurs owing to the mutual repulsion between ions, expanding the rearward portion of the beam 53. Provided the decelerating electrodes 52 have been suitably positioned, the outermost ions in the beam 53 will strike the electrodes '52 at a highly oblique angle. Since many more secondary electrons are released as a secondary emitting surface is struck at an increasingly oblique angle, the configuration of the electrodes 52 provides a highly efiicient source of secondary electrons as indicated by dotted lines 56. The positive charge of the ions attracts the negative electrons and the result is neutralization of the former. As in the embodiments of the invention heretofore described, the resultant charge on the rocket frame is automatically regulated since more or less electrons return to the decelerating electrodes as the static negative charge on the rocket frame is respectively lesser or greater. The minus 50,000 volts applied to the accelerating electrode 51 provides apotential barrier which isolates the porous electrode 24 from the source of electrons 52, thereby preventing a power loss. Considering further that no filament power is dissipated, the embodiment of the invention shown in FIGURE 5 is more efficient than the prior forms. 1

With reference to all the described embodiments of the invention, a typical dimension for the distance across one concave groove of the porous electrode 24 is 0.25 inch which dimension is partially governed by the necessity of 'preventing'arcing between adjacent charged electrodes. "For a space vehicle of moderate size the area of the ion 5 emitting surface may be a total of four square feet. Thus, as shown in FIGURE 1, a large number, sixty four for example, of the individual sections as described in conjunctionwith- FIGURES 2 to 5, inclusive, may be required; Since the individual sections 20 may be sepa- 'rately" controlled, operation of any selected section or group thereof may be discontinued without affecting the remaining sections in the event of damage or malfunctioning. In a similar manner, individual control over the sections provides a mechanism for guidance. 1 Numerous variations in the described structure are poslsiblewithinthe scope of theinvention. The heating of "the" propellant in vessel 12, as-well as the porous electrode for example, may be provided for by coolant from a nu- ,clear reactor instead of by the electrical heating utilized 'in the' described embodiments. While cesium is a highly satisfactory and efficient propellant, other substances such as rubidium, or potassium are also usable. Similarly, the porous electrode used for ionizing the propellant, is preferably tungsten but porous molybdenum as well as other materials may also be utilized.

Accordingly, while the invention has been disclosed with respect to certain exemplary embodiments, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In apparatus for generating an ion beam, the combination comprising an electrode formed of a porous material, means forcing an ionizable substance under pressure against a first side of said electrode, at least one ion accelerating electrode spaced from the opposite side of said porous electrode, and an electrical voltage source having connections for applying a negative potential to said accelerating electrode relative to said porous material for establishing an electrical field therebetween.

2. In apparatus for producing an ion beam, the combination comprising a vessel containing an ionizable substance, said vessel having a wall portion characterized by minute porosites which communicate from the interior to the exterior of the vessel, means for maintaining said porous wall portion at an elevated temperature, at least one accelerating electrode spaced from said wall portion, and an electrical power supply applying a negative potential to said accelerating electrode relative to said wall portion whereby ions emitted therefrom are accelerated outwardly therefrom.

3. In an ion beam generating apparatus, the combination comp-rising a gas channel having a terminal wall formed of metal characterized by minute porosities which communicate between the interior and exterior surfaces thereof, a heating element in thermal contact with said wall, a plurality of accelerating electrodes spaced from said wall and mutually spaced apart, and an electrical potential source applying a negative charge to said accelerating electrodes relative to that of said wall whereby ions emitted therefrom are substantially unidirectionally accelerated.

4. An ion beam generating apparatus substantially as described in claim 3 and comprising the further combination of a source of vaporized pressurized cesium communicating with said gas channel and wherein said metal forming said terminal wall is selected from the group: tungsten, platinum, molybdenum, tantalum and carbon.

5. An ion beam generator particularly suited for rocket propulsion comprising, in combination, a vessel having a substantial wall area formed of metal characterized by small porosities which communicate between the interior and exterior of the vessel, a reservoir containing a vaporizable and ionizable propellant substance and having an outlet conduit communicating with said vessel, a first heating means associated with said reservoir for vaporizing and pressurizing said propellanthsubstance therein, a second heating means associated with said porous wall area of said vessel for causing ionization of propellant passing therethrough, a plurality of spaced apart linear parallel accelerating electrodes uniformly spaced from said porous wall area of said vessel, and an electrical power supply connected between said accelerating electrodes and said porous wall area of said vessel and applying a negative potential to said electrodes relative to said wall area for establishing an electrical field therebetween.

6. An ion beam generator particularly suited for rocket propulsion comprising, in combination, a vessel having a substantial wall area formed of metal characterized by small porosities which communicate between the interior and exterior of the vessel, said wall area being further characterized by a plurality of parallel grooves formed on the outer surface thereof, a reservoir containing a vaporizable and ionizab-le propellant substance and having an outlet conduit communicating with said vessel, 21 first heating means associated with said reservoir for vaporizing and pressurizing said propellant substance therein, a second heating means associated with said porous wall area of said vessel for causing ionization of propellant passing therethrough, a plurality of spaced apart linear parallel accelerating electrodes uniformly spaced from said porous wall area of said vessel and being parallel to said grooves thereof and situated one between each adjacent pair of said grooves, and an electrical power supply connected between said accelerating electrodes and said porous wall area of said vessel and applying a negative potential to said electrodes relative to said wall area for establishing an electrical field therebetween which field is curved by said grooves to cause ions to pass between said accelerating electrodes.

7. An ion beam generator substantially as described in claim 5 and comprising the further combination of a magnetic field generating element disposed adjacent each of said accelerating electrodes and aligned to provide a magnetic field which is normal to the plane of said electrodes in the region thereof whereby ions are constrained to pass between said electrodes.

8. In a rocket propulsion system, the combination comprising an electrode having porosities which pass from a forward to a rearward side thereof, a gas conduit supplying ionizable vapor to said forward side of said porous electrode, means heating said porous electrode to ionize vapor passing therethrough, a plurality of accelerating electrodes spaced rearwardly from said rearward side of said porous electrode, a voltage source connected to said accelerating electrodes and applying a negative potential thereto relative to said porous electrode for accelerating ions emerging therefrom, and an electron emitting element disposed rearwardly from said porous electrode for neutralizing said ions.

9. An ion rocket engine comprising, in combination, a porous electrode of substantial surface area, means supplying pressurized ionizable gas to a forward side of said porous electrode for transmission therethrough, a heating element in thermal contact with said porous electrode, a plurality of spaced apart accelerating electrodes disposed rearwardly from said porous electrode, a voltage source connected to apply a negative potential to said accelerating electrodes relative to said porous electrode, and a plurality of electron emissive elements disposed rearwardly trom said porous electrode and rearwardly from at least a portion of each said accelerating electrode in order to be shielded from said porous electrode thereby.

10. An ion rocket engine substantially as described in claim 9 wherein said accelerating electrodes are semicircular in cross-section with the open side thereof facing rearwardly, said plurality of electron emissive elements comprising electrically heated filaments one disposed centrally within each said accelerating elect-rode.

11. An ion rocket engine substantially as described in claim 9 wherein said electron emissive elements comprise a material of high secondary electron emissiveness positioned in the path of ions accelerated rearwardly from said porous electrode and comprising the further combination'of means applying an electrical potential to said electron emissive elements which potential is intermediate between that of said porous electrode and said accelerating electrodes.

12. in a thrust producing engine for an ion rocket, the combination comprising a propellant channel having a terminal wall portion formed of gas pervious metal, the rearward exterior surface of said wall portion having at least one concavity formed therein, an accelerating electrode spaced rearwardly from said exterior surface of said wall portion, said accelerating electrode being shaped to conform to the margin of said concavity and being substantially in alignment therewith, a voltage source ap lying a negative potential to said accelerating electrode relative to said wall portion for withdrawing a beam of ions O o therefrom, and an electron emissive element positioned rearwardly from at least the forward portion of said accelerating electrode and emitting electrons in the direction of said ion beam to neutralize the charge thereof.

13. An ion propelled rocket engine comprising, in combination, a vessel having a rearward gas pervious wall formed of porous metal the exterior surface of said wall having a plurality of parallel grooves 'formed thereon, a propellant supply reservoir having provision for vaporizing and pressurizing an ioniza'ble propellant substance contained therein and having an outlet conduit communicating with said vessel, a heating element in thermal contact with said gas pervious wall for maintaining an elevated temperature thereof to ionize propellant passing therethrough, a plurality of electron emitting elements spaced directly rearward from the margins of said grooves in said gas pervious wall in parallel relationship to said margins, a plurality of accelerating electrodes spaced directly rearward from said margins of said grooves and in parallel alignment therewith, at least a portion of each said accelerating electrode being positioned forwardly from said electron emitting elements, an electrical power supply connected to apply a negative potential to said accelerating electrodes relative to said gas pervious wall wheerby ions emerging therefrom are shaped into a rearwardly directed beam, said ions being subsequently neutralized by said electrons.

14. An ion propelled rocket engine substantially as described in claim 13 and comprising the further combination of a plurality of magnets, one of said magnets being secured to each of said accelerating electrodes and each having a first polarity along the forward edge and an oppostie polarity along the rearward edge whereby a magnetic field is established in the region of said accelerating electrodes which field tends to guide said ions midway between adjacent ones of said electrodes.

15. In an ion rocket, a thrust generator comprising in combination a vessel having a rearward wall formed by a porous tungsten plate the exterior surface of which wall is provided with a plurality of linear parallel grooves, a heating element in thermal contact with said plate, a reservoir containing cesium metal and having an outlet communicating with said vessel, a second heating element in thermal contact with said reservoir for vaporizing and pressurizing said cesium therein, a plurality of accelerating electrodes of linear configuration one spaced directly rearward from the boundary between each neighboring pair of grooves on said plate and in parallel alignment with said grooves, a voltage source applying a negative potential to said accelerating electrodes with respect to said plate, and a plurality of linear electron emissive members one disposed rearwardly from at least the forward portion of each said accelerating electrode and in parallel relationship thereto whereby ions emerging from said plate and accelerated by the potential on said electrodes are neutralized.

References Cited in the tile of this patent UNITED STATES PATENTS 2,499,289 Backus Feb. 28, 1950 2,736,809 Bacon Feb. 28, 1956 2,880,337 Langmuir et a1. Mar. 31, 1959 OTHER REFERENCES Nuclear ion Rocket, SAE Journal, vol. 67, July 1959, pp. 40.

Disclaimer 3,014,154.Kenmeth We Ehlews, Lafayette, and Ferdinand VoeZlcer Ill, Concord, Calif. ION ROCKET ENGINE. Patent dated Dec. 19, 1961. Disclaimer filed Mar. 8, 1965, by the assignee, United States of Ameflea as e'epresented by the United States Atomic Enemy 00mmtsszon.

Hereby enters this disclaimer to claims 1, 2, 3, 4, 5, 6, 8, 9, 12, 13 and 15 of said patent.

[Ofiiez'al Gazette May 4, 1965.]

Disclaimer 3,014,154.Kenmeth W; Ehlene, Lafayette, and Ferdinand VoeZlcer Ill, Concord, Calif. ION ROCKET ENGINE. Patent dated Dec. 19, 1961. Disclaimer filedv Mar. 8, 1965, by the assignee, United States of America as represented by the United States Atomic Energy Own- 4 Hereby entere thi disclaimer to claims 1, 2, 3, 4, 5, 6, 8, 9, 12, 13 and. 1 5 n of said patent.

[Oficz'al Gazette May 4, 1965.] 

